Without limiting the scope of the present invention, its background will be described in relation to a downhole safety joint, as an example.
There are many different operations involved in drilling and completing an oil and/or gas well; some of these operation include drilling, surveying, and completing a well. Oftentimes, these wells are drilled at extreme depths and many times they are drilled directionally such that one or more bends exist in the wellbore that can cause a pipe string, drillstring, tool string, service string, and the like (“work string”) to become stuck deep in the wellbore. Many times expensive tools, instruments, and the like are located towards the lower end of these work strings. Thus, once stuck, it oftentimes is desirable to retrieve as much of this equipment and instruments as possible.
One method for recovering this equipment involves running a string shot on a wireline down as far as possible through the inner diameter of the stuck work string and firing an explosive to separate the joint where it can be backed off. Typically, this process includes putting left hand torque on the work string, applying substantially neutral weight to the desired joint proximal to the string shot, and then firing the string shot, which causes the joint to break enabling the recovery of the work string and any equipment and instruments above the joint to be recovered. One of the problems associated with this procedure is that many times the work string may include equipment, tools, instruments, and their related components disposed in the inner diameter thus blocking the downward passage of the string shot and wire line past that point that would prevent locating and firing the string shot below that point. Any expensive equipment and instruments located below that point would not be able to be retrieved typically using this method.
Another retrieval method is to include what is known as a “safety joint” in the work string. A safety joint is typically a tubular member consisting of an upper and lower sub that are disconnectable from each by a variety of known means. In one such means, coarse threads join the upper and lower sub, such that when a string becomes stuck in a wellbore, left hand torque may be applied to the work string which then uncouples (unscrews) the upper sub from the lower sub, thus enabling the upper sub and the work string above it to be retrieved leaving the lower sub and parts of the work string below it in the wellbore. Typically, the torque required to unscrew the safety joint is a fraction of the torque required to break the threaded connections between the joints of the work string, which the safety joint is connected, thus unscrewing the safety joint but not any other tubular members of the work string. Sometimes, these safety joints are placed lower in the work string than expensive equipment and instruments, thus ensuring that the equipment and instruments may be retrieved once the safety joint has been disconnected.
Also, once retrieved at the surface and the expensive equipment and instruments have been recovered, the upper sub may be re-coupled to a work string having a substantially open inner diameter, and run back into the wellbore for reconnecting with the lower sub. Doing so then provides a substantially open inner diameter all the way to the bottom hole assembly (“BHA”) at or near the bottom of the wellbore or distal end of the stuck work string. This method may then include running a string shot in and shooting it off to recover more of the stuck work string via a wireline or other known means. In another method, a jar may be attached upstring of the retrieved upper sub and run back into the wellbore for reconnecting with the lower sub of the safety joint and jarring the stuck work string.
One problem associated with these types of safety joints is that the threaded sections of the subs making up a break joint may include seals disposed about the ends of the threaded sections that may trap fluids or mud within the safety joint when the upper sub is being reconnected with the lower sub in the wellbore. The trapped mud or fluid located within the upper and lower subs is under extreme pressure and may cause the subs to become hydraulically locked. Drilling mud is often designed to fill and plug voids to prevent fluid loss into the formations being penetrated by the wellbore. This characteristic can cause difficulty in making up a safety joint downhole because the mud tends to plug off and seal inside the threads as they are screwed back together. This can further add to the problem of hydraulic locking in the safety joint because the fluid is trapped inside the threaded connection and cannot be exhausted through the safety joint.
When hydraulically locked, operators may apply more torque in response to the hydraulic lock in an attempt to reach a proper seat of the upper and lower sub, which may damage the safety joint, subs, and/or other equipment in the wellbore.
Another problem associated with hydraulically locked subs is that when torque is backed off due to the operator's belief that the threaded ends of the subs are properly engaged, it will in fact mean that the safety joint is not properly made up and may become disconnected when it is retrieved from the wellbore, thus causing tubular members, equipment, instruments, and the like to be dropped into the wellbore.
Additionally, conventional safety joints are oftentimes run into wellbores having highly deviated, horizontal, or tortuous trajectories to access substantially horizontal hydrocarbon bearing formations. Under these situations, the safety joint experiences a tensile load (e.g., pulling work string out of wellbore) or a compressive load (e.g., adding weight to the work string) in the axial direction of the safety joint while in the wellbore. In addition, the safety joint will experience a bending or side load when it is in these situations or environments. These bending loads are caused by the distal ends of the safety joint being in contact with a sidewall of the wellbore, casing, liner, etc., while concurrently the substantially opposite side of the safety joint's central section or break joint encounters a substantially opposite linear side load. The side load creates a compressive stress on one side of the break joint and a tensile stress on the opposite side of the break joint.
Further, the stress caused by the axial loading will add to or subtract from the stress caused by the bending load. If there is a large enough positive or negative axial load, the safety joint will remain completely or constantly in tensile or compressive stress throughout the safety joint, but the sides or top/bottom (substantially horizontal orientation) of the safety joint will experience different stress levels due to the bending load or stress. It is this cyclical variation in stress state caused by the cyclic bending loads that causes break joints to tighten, loosen, cause total failure of the break joint. Also, the shoulders of the break joint may become damaged by the cyclical loading causing the break joint to become looser than required, thus causing unreliable break joint connections that are difficult to reliably make up or break under desired torque ratings.